The present invention relates to methods and devices used to control electric motors. More particularly, the invention relates to a method and an apparatus to automatically reverse an electric motor to control the operation of a tool, such as a dispensing gun, driven by the motor.
Hand powered dispensing guns have been available for many years. More recently, pneumatic and electric versions of hand-powered dispensing guns have been made available. Most dispensing guns, whether hand-powered, pneumatic, or electric, operate in a similar manner. A cartridge of material is placed in a cradle of the gun. The cartridge has a nozzle on one end and a disk or back wall on the other. The gun includes a plunger that is positioned coaxially with the back wall when the cartridge is placed in the cradle. The plunger contacts the back wall and moves it forward forcing the material in the cartridge out of the nozzle.
Electric-corded and battery-powered dispensing guns include an electric motor controlled by a trigger-actuated switch. Pulling the trigger closes the switch and electrically interconnects the motor to the power source. The motor drives a rack and pinion mechanism to advance the plunger. The plunger is thereby driven into the back wall of the cartridge to dispense the material. The operator can control the advancement of the plunger into the material cartridge and, thereby, control the application of material to the desired surface. Powered dispensing guns typically incorporate a speed control mechanism that allows the operator to control the speed of the flow of material out of the cartridge nozzle. As the operator becomes proficient with the tool, he or she can increase the speed at which the material is dispensed. The operator typically also has the option of retracting the plunger from the back wall of the cartridge. Operators remove the plunger from the cartridge when the cartridge is empty or when the job has been completed. Some dispensing guns have a manual switch to change the direction of the motor, thereby changing the direction of the plunger and retracting it away from the cartridge. Other dispensing guns have a manual mechanical release that allows the operator to physically pull the plunger out of contact with the cartridge.
While present dispensing guns are functional, they suffer from at least one deficiency. In general, once material is dispensed from a dispensing gun the material continues to flow out of the gun""s nozzle after the gun""s trigger mechanism has been released. After-flow (or oozing at the dispensing tip) leads to waste of material, nozzle fouling, dropped material, and additional clean up time.
Two primary reasons for the after-flow phenomenon are recognized. First, the usually thin-walled cartridge expands during plunger actuation and, according to the physical law that systems always attempt to return to the relaxed state, the cartridge wall relaxes after the plunger actuation. Since the back walls of most cartridges are designed to retain their forward-most position and the plunger of the dispensing gun is typically locked against a return movement, the relaxation of the cartridge wall leads to after-flow. Second, most dispensed compositions have a high viscosity and are at least marginally compressible. Thus, plunger actuation usually causes a substantial internal pressure buildup in the cartridge that, after the plunger is no longer forced forward, results in material leaking from the nozzle tip.
Accordingly, it would be desirable to have an improved method and device to control after-flow in a dispensing gun. The invention provides a method and apparatus to automatically reverse the motor of a dispensing gun for a predetermined amount of time. An electronic motor controller automatically reverses the direction of plunger movement and removes the plunger from contact with the back wall of the cartridge. The automatic reversal of the motor to reverse plunger motion alleviates after-flow problems. The electronic motor controller includes a trigger switch, a power supply circuit, a commutator, an overload sensor, and a programmable device.
The trigger switch is coupled to a power source, such as a battery, and includes a main power on/off switch and a potentiometer. A protection or secondary switch is coupled in parallel to the main switch. The power supply circuit and the commutator are each coupled to the main and secondary switches. The overload sensor is coupled to the commutator and the programmable device. The programmable device is coupled to the power supply circuit, the potentiometer, the commutator, and the overload sensor. The programmable device is operable to sense actuation and deactuation of the main power on/off switch, read an electromotive force from the potentiometer, and, upon sensing deactuation of the main switch, send a control signal to the commutator to reverse current flow therethrough for a predetermined amount of time. The programmable device also deactuates the secondary switch when the predetermined time has lapsed. Reversing the direction of the motor upon release of the trigger switch reverses the direction of the plunger and stops forward movement of the back wall of the cartridge in the dispensing gun. As noted, automatically stopping the forward movement of the back wall alleviates after flow problems.
The invention may also be implemented in a method including sensing actuation of the trigger switch, reading a voltage from the trigger switch, generating a first control signal if the voltage from the trigger switch is equal to or greater than a predetermined value, sending the first control signal to a commutator to drive an electric motor in a first direction, generating a second control signal if the voltage from the trigger switch is equal to or less than a predetermined cut-off value, sending the second control signal to the commutator to drive the electric motor in a second direction, opposite the first direction, for a predetermined amount of time, and deactuating the secondary switch when the predetermined time has lapsed.
As is apparent from the above, it is an advantage of the present invention to provide an electronic motor control to automatically reverse a motor to prevent after-flow of material from a tool such as a dispensing gun. Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings.